The long-term goal of this research is to understand how the nervous system combines information from multiple senses for the control of multisegment human upright stance. Imbalance is a major cause of fallrelated injuries, whose cost to the health-care system is expected to exceed $32B by 2020. Current interventions to improve balance and reduce the risk for falls lack a theoretical framework because the mechanisms by which they affect postural control processes are not well understood. Until the control processes underlying a balance deficit are understood, rehabilitative programs will continue to intervene in costly and time-consuming non-specific ways. This proposal emphasizes an experimental strategy designed to probe the underlying postural control loop in patient populations with far more certainty than present methods. Ten experiments will investigate three specific aims. I. To identify open-loop frequency response functions characterizing multijoint postural control. II. To investigate the dynamics of intermodality reweighting. III. To investigate the adaptability of the multijoint control strategy in bilateral vestibular loss (BVL) patients. We propse to build on the findings from previous grant cycles focusing on the properties of multisensory reweighting by manipulating different combinations of two sensory inputs. In addition, we will expand a system identification technique previously developed for single-joint postural control. Multiple, simultaneous, mechnical and sensory perturbations will identify how muscular activity translates into control of body segment positions (the plant) and how, in turn, body segment positions translate into new muscular activity (feedback). This identification will be achieved within the context of the multi-joint body, whose relative configuration must be incorporated into the estimate of body dynamics. Finally, healthy and bilateral vestibular loss individuals will be compared with these identification techniques to determine how loss of vestibular function disrupts the plant and feedback control loops. With such knowledge, rehabilitative efforts may then emphasize methods that focus on a particular process within the control loop to optimize rehabilitation, leading to more effective, targeted care and reduction of health care costs.